Method and apparatus for butt welding together sheet blanks

ABSTRACT

An apparatus is disclosed for positioning, clamping and welding together proximal edge portions of two sheet blanks. The apparatus incorporates a high energy laser and sheet clamping mechanism used to clamp each sheet blank in place between an associated electromagnet and ferromagnetic clamping shoes. The shoes are positioned above a corresponding electromagnet and are vertically movable there towards on the activation of the electromagnet. In use, the sheets are held in place with the respective sheet blanks sandwiched between the associated electromagnets and shoes with the proximal edge portions of the sheets abutting. The high energy laser is movably provided in the apparatus to move an emitted laser beam along a predetermined path over the abutting proximal edge portions to perform welding operations.

SCOPE OF THE INVENTION

The present invention relates to a method and apparatus for aligning andwelding together two or more sheet blanks and, more particularly, to anapparatus adapted to automatically align and butt weld together proximaledge portions of two sheets along a common seam line.

BACKGROUND OF THE INVENTION

Present day manufacturing requirements often necessitate the formationof various workpiece components by welding together two or more metalsheet blanks. Typically, individual sheet blanks are joined byconventional seam or butt welding equipment, such as high energy CO₂lasers, or electric beam/plasma arc-welding devices. To obtain aneffective and complete weld, it is necessary to ensure that the proximaledges of the sheets to be joined are positioned and maintained in adirectly abutting orientation during the welding operation.

Where CO₂ lasers are used to perform welding operations, to ensure theformation of a complete weld seam, it is necessary that the edges of thesheet blanks be pre-finished and have a mirror-smooth finish.

Conventional welding apparatus used to join sheet blanks incorporatemechanical clamps for holding the sheets together. The use of mechanicalclamps to secure the sheet blanks during welding, however, leads toinefficiencies in workpiece production. In particular, mechanicalclamping mechanisms require increased time to clamp and unclamp thesheet blanks in position prior to and after welding. A furtherdisadvantage of conventional apparatus is that the increased clampingtime resulting from the use of mechanical clamps inhibits adjustment inthe positioning of the proximal edges of the sheets which are to bewelded together once the clamps are engaged.

Mechanical clamps which are used in conventional welding apparatus aretypically pneumatically or hydraulically operated, requiring that thewelding apparatus include large rigid and expensive clamp supportingframes to absorb the clamping forces.

In one attempt to overcome the disadvantages of prior art weldingapparatus, magnetic clamping devices have been proposed, as for example,is disclosed in U.S. Pat. No. 5,023,427 which issued Jun. 11, 1991 toNeiheisel et al. The Neiheisel patent discloses an apparatus foraligning steel sheets which are to be welded together by a laser. In theapparatus of Neiheisel, an electromagnet is used to support the sheetsin such a manner as to induce opposite magnetic polarities in theproximal edges of adjacent sheets, drawing the sheets together bycreating a magnetic attraction therebetween.

While the apparatus of Neiheisel may be useful in aligning proximal edgeportions of iron or other ferromagnetic sheets which are of a thicknesssufficient to minimize magnetic reluctance therebetween, the apparatusof Neiheisel is poorly suited for use in aligning thinner steel sheetblanks, or sheet blanks which are plated or coated or which are madefrom non-ferromagnetic materials such as copper, aluminum, plastic andthe like.

SUMMARY OF THE INVENTION

To at least partially overcome the difficulties of prior art weldingapparatus, the present invention incorporates a sheet clamping mechanismwhich clamps sheet blanks in place between an electrically activatedmagnetic sheet supporting surface, and at least one clamping shoe whichis magnetically attracted to the magnet. The shoe is positioned abovethe magnetic surface and is vertically movable theretowards with theactivation of the magnetic surface, whereby the magnetic attraction ofthe shoe towards the magnetic surface pulls the shoe downwardly. In thismanner, part of the sheet blank which is sandwiched between the shoe andmagnetic surface is held in place at least in part by the downward forceapplied by the magnetic attraction of the shoe to the magnetic surface.The electromagnetic clamping mechanism of the present inventionadvantageously permits almost instant clamping and unclamping of thesheets to be welded by activating and deactivating the electromagnet.Further as clamping is performed by the magnetic attraction of the shoe,the apparatus may be used to weld non-ferromagnetic sheet blanks.

Another object of the invention is to provide an apparatus foraccurately and quickly aligning the proximal edge portions of two ormore sheets which are to be welded together in a position aligned withthe path of a movable welding apparatus.

It is another object of the invention to provide an apparatus adapted toalign and secure in an abutting relationship the proximal edge portionsof both ferromagnetic sheet blanks and non-ferromagnetic sheet blanks,as for example may be made from iron, steel, copper, aluminum, nickel,metal alloys and/or plastics and the like.

Another object is to provide an apparatus for laser butt welding two ormore sheet blanks together, without requiring that the edge portions ofthe blanks be pre-finished.

A further object of the invention is to provide a simplified apparatusfor automatically aligning and welding the proximal edges of sheetblanks to be joined, which may easily be adapted to perform weldingoperations on different sheet blanks to produce different finishedworkpieces.

Another object of the invention is to provide an apparatus having aclamping mechanism for clamping together edge portions of workpieceblanks, and which does not require a large or cumbersome supportingframe to offset and absorb clamping forces.

Another object of the invention is to provide an apparatus forpositioning and welding together proximal edge portions of two or moresheet blanks having different relative thicknesses.

To achieve at least some of the foregoing objects, the present inventionincludes a laser or other welding apparatus for welding togetherproximal edge portions of two sheet blanks, and two clamping mechanisms,each supporting thereon an associated respective one of the sheet blanksto be joined in a desired orientation relative to the other. Preferably,a high energy laser, such as a yttrium aluminum garnet (YAG) laser, isused to weld the blanks. The laser is movably provided in the apparatusso as to move an emitted laser beam relative to the sheet blanks along asensed or predetermined linear and/or curved path. In this manner, thelaser is activated and moved along its sensed/predetermined path to weldproximal edges of the two sheet blanks together along a seam linealigned with the path of the laser beam.

At least one clamping mechanism includes an electrically activatedmagnetic supporting surface which supports a lower side of theassociated sheet blank thereon, and one or more vertically movableclamping shoes spaced above the magnetic supporting surface. Themagnetic surface is preferably part of an electromagnet operated byelectric current. The electromagnet may comprise of one or moreconventional electromagnets, but more preferably consists of one or aseries of electronically switchable permanent magnets which are arrangedin an array and contain electric coils, and which are electricallypulsed to provide increased downward magnetic forces.

The clamping shoes are made from a ferromagnetic material such as iron,or is otherwise constructed having a high magnetic susceptibility so asto be magnetically attracted and held down by the electromagneticsupport when the support is activated. By lowering the shoes of eachclamping mechanism onto the top of the sheet blank so it is sandwichedbetween the support and shoes, the proximal portions of the sheets areclamped in position. Clamping is achieved by both the downward forceapplied by the shoe as a result of its magnetic attraction to thecorresponding electromagnetic support as well as by any magneticattraction of the sheet blank to the support.

The electromagnetic support may be provided as an elongatedelectromagnet which extends longitudinally along and supports theunderside of a respective sheet, adjacent the proximal edge to bejoined. The shoe may also be formed as a single elongate member whichextends longitudinally above the electromagnetic support, or for ease ofmanufacture a number of smaller, longitudinally spaced shoes areprovided at spaced locations thereabove.

The shoes are mounted to the apparatus, so as to be reciprocally movablebetween a raised position spaced slightly above the upper surface of thesheet, and a lowered clamping position. In the clamping position, theshoe is moved downwardly into engagement with the upper surface of thesheet, clamping it against movement between the shoe and the respectivesupporting electromagnet.

A positioning guide may also be provided to ensure correct location ofthe sheet blanks. The guide acts to locate the edge proximal portion ofone of the sheet blanks in a position aligned with the predeterminedseam line. When the proximal edge of the second other sheet is movedagainst the other sheet, the proximal edge portions of both sheets thusboth orient in an abutting relationship aligned with the predeterminedseam line.

Accordingly in one aspect the present invention resides in an apparatusfor aligning and welding together proximal edge portions of first andsecond sheets, said apparatus including,

welding means for welding the proximal edge portion of the first sheetto the proximal edge portion of the second sheet along a seam line,

positioning means for positioning the proximal edge portion of saidfirst sheet in a welding position substantially aligned with said seamline,

first sheet supporting means for supporting said first sheet with saidedge portion of said first sheet in said welding position, the firstsheet supporting means including,

first clamping means for releasably retaining said first sheet in saidapparatus during welding, the first clamping means comprising,

first magnetic hold-down means for engagingly supporting a first side ofsaid first sheet, and

first shoe means for engaging the second other side of said first sheet,the first shoe means characterized by magnetic susceptibility and beingmovable relative to said first hold-down means between a forwardposition wherein said first shoe means is moved towards said firsthold-down means a distance selected to substantially prevent movement ofsaid first sheet therebetween and a rearward position wherein said firstshoe means is moved away from said first hold-down means to permitsubstantially unhindered movement of said first sheet therebetween,

the first clamping means being activatable to produce a magnetic fieldin the first hold-down means sufficient to magnetically attract thefirst shoe means to the forward position and clamp the first sheettherebetween, and

second sheet supporting means for supporting said second sheet with saidproximal edge portion of said second sheet in a generally abuttingrelationship with said proximal edge portion of said first sheet.

In a further aspect the present invention resides in an apparatus forforming a workpiece by welding together proximal edge portions of firstand second sheet blanks, said apparatus including,

welding means for welding the proximal edge portion of the first sheetto the proximal edge portion of the second sheet along a seam line,

positioning means for maintaining the edge portion of said first sheetin a position substantially aligned with said seam line,

first sheet supporting means for supporting said first sheet in agenerally horizontal position thereon with said edge portion of saidfirst sheet in said position aligned with said seam line, the firstsheet supporting means including,

first clamping means for releasably retaining said first sheet in saidapparatus during welding the first clamping means comprising,

first magnetic hold-down means for engagingly supporting a first lowerside of said first sheet, and

first shoe means for engaging the other upper side of said first sheet,the first shoe means characterized by magnetic susceptibility movablerelative to the first hold-down means between a sheet engaging position,wherein the first shoe means is lowered towards the first hold-downmeans to engage and clamp the first sheet in position between the firstshoe means and first hold-down means, and a release position wherein thefirst shoe means is raised above the first hold-down means a distanceselected to permit movement of the first sheet therebetween,

wherein the clamping means is activatable to produce a magnetic field inthe first hold-down means to magnetically attract the first shoe meansand assist in moving the first shoe means to the sheet engagingposition,

second sheet supporting means for supporting said second sheet thereonin a substantially horizontal orientation with said edge portion of saidsecond sheet in an approximately abutting relationship with said edgeportion of said first sheet.

In another aspect the present invention resides in a use of an apparatusto form a workpiece by welding together proximal edge portions of firstand second sheet blanks, said apparatus including,

welding means for welding the proximal edge portion of the first sheetto the proximal edge portion of the second sheet along a predeterminedor sensed seam line,

positioning means for positioning the edge portion of said first sheetin a position substantially aligned with said seam line,

first sheet supporting means for supporting said first sheet with saidedge portion of said first sheet in said position aligned with said seamline, the first sheet supporting means including,

first clamping means activatably to releasably retain said first sheetin said apparatus during welding, the first clamping comprising,

first electromagnetic hold-down means for engagingly supporting a firstside of said first sheet, and

first shoe means for engaging the second other side of said first sheet,the first shoe means characterized by magnetic susceptibility and beingmovable relative to said first hold-down mans between a forward clampingposition wherein said first shoe means is moved towards said firsthold-down means to engage and clamp a portion of said first sheettherebetween, and a rearward position wherein said first shoe means ismoved away from said first hold-down means a distance sufficient topermit movement of said first sheet therebetween, and

second sheet supporting means for supporting said second sheet thereonwith said edge portion of said second sheet in an approximately abuttingrelationship with said edge portion of said first sheet, said secondsheet supporting means including,

second clamping means activatable to releasably retain said second sheetin said apparatus during welding, the second clamping means comprising,

second electromagnetic hold-down means for engagingly supporting a firstside of said second sheet, and

second shoe means for engaging the second other side of said secondsheet, the second shoe means characterized by magnetic susceptibilityand being movable relative to said second hold-down means between aforward clamping position, wherein said second shoe means is movedtowards said second hold-down means to engage and clamp a portion ofsaid second sheet therebetween, and a rearward position wherein saidsecond shoe means is moved away from said second hold-down means adistance sufficient to permit movement of said second sheettherebetween,

wherein with each of said first and second means positioned in saidrespective rearward positions, said workpiece is formed by the steps of

moving said first sheet between said first shoe means and said firsthold-down means to engage said positioning means and to position saidedge portion of said first sheet substantially in alignment with saidseam line,

activating said first clamping means to product a magnetic field in saidfirst hold-down means and move the first show means to the forwardposition to retain the first sheet therebetween,

moving said second sheet between said first shoe means and said secondhold-down means to position the edge portion of the second sheet in aposition abutting the edge portion of the first sheet,

activating the second clamping means to produce a magnetic field in saidsecond hold-down means and move the second shoe means to the forwardposition to retain the second sheet therebetween, and

activating said welding means to perform welding along the seam line toweld together the edge portion of the first sheet and the edge portionof the second sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the invention will appear from thefollowing description taken together with the accompanying drawings inwhich:

FIG. 1 is a cross-sectional end view of a welding apparatus inaccordance with a first preferred embodiment of the invention;

FIG. 2 is a top view of the apparatus of FIG. 1 showing the formation ofa workpiece composite blank thereon;

FIG. 3 is a cross-sectional view of the apparatus as shown in FIG. 2taken along lines 3-3';

FIG. 4 is an enlarged partial cross-sectional view of the weldingapparatus of FIG. 1 taken along line 4-4', showing the insertion of afirst sheet blank to be welded;

FIG. 5 is a cross-sectional view of the welding apparatus as shown inFIG. 4 with an edge portion of the first sheet aligned at apredetermined seam line;

FIG. 6 is a cross-sectional view of the welding apparatus as shown inFIG. 4 showing the insertion of the second sheet blank thereto;

FIG. 7 is a cross-sectional view of the welding apparatus as shown inFIG. 4 showing the proximal edge portions of the first and second sheetsin an abutting position aligned with the predetermined seam line;

FIG. 8 shows a partial schematic side view of a clamping mechanism inaccordance with a further embodiment of the invention;

FIG. 9 shows a schematic end view of the welding apparatus in accordancewith a second preferred embodiment of the invention;

FIG. 10 shows a schematic top view of the apparatus of FIG. 9 showingthe formation of a composite workpiece blank thereon;

FIG. 11 shows a schematic top view of a production assembly line forforming composite workpieces in accordance with a third embodiment ofthe invention;

FIG. 12 is a partial sectional side view of the welding apparatus shownin FIG. 11 taken along line 13-12' and showing the laser apparatus usedto weld sheet blanks;

FIG. 13 shows a schematic top view of a production assembly line forforming composite workpiece blanks in accordance with a fourthembodiment of the invention; and

FIG. 14 shows a schematic side view of the laser welding apparatus shownin FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made to FIG. 1 which shows an apparatus 10 for aligning andwelding together two planar steel sheets 12,14. The apparatus 10 joinsthe sheets 12,14 together along a longitudinally extending andpredetermined linear seam line 16 which is aligned with the longitudinalaxis A₁ of the apparatus 10, to form a finished composite workpiece 18(shown best in FIG. 2). As will be described hereafter, the apparatus 10joins the sheets 12,14 by welding their respective proximal or facingedge portions 12',14' together. It is to be appreciated that the sheets12,14 are formed having generally linear proximal edge portions 12',14'which are complementary to each other so as to properly meet along thedesired seam line 16, with each edge portion 12',14' to be joinedextending generally in the direction of axis A₁.

To move the sheet blanks 12,14 into the apparatus 10 for welding, theapparatus 10 is provided with two magnetic stepping conveyors 20,22which extend longitudinally along each side of the axis A₁. A sheetpositioning assembly which includes three cylindrical weld datum orlocating pins 30 (shown best in FIGS. 4 to 7) and a number of spacedpneumatically operated positioning slide assemblies or rods 32,34 isused for the final alignment of the blanks 12,14 prior to welding. Aswill be described in detail hereafter, two independently operable sheetclamping units 36,38 secure a respective sheet 12,14 in position duringwelding so that the proximal edge portions 12',14' of the sheets 12,14are maintained in the proper orientation aligned with the predeterminedseam line. A Lumonics 2 kW YAG laser is provided for welding the sheets12,14 together along the predetermined workpiece seam line 16.

As seen best in FIGS. 1 and 2, the magnetic conveyors 20,22 are providedin a spaced apart arrangement each extending parallel to and on eachside of the seam line 16. The conveyors 20,22 are independently operableto move a respective sheet blank 12,14 in the longitudinal direction ofarrow 42 (FIG. 2) into the apparatus 10 to a workstation for welding; aswell as moving the finished workpiece 18 outwardly therefrom. Althoughnot essential, the use of magnetic conveyors 20,22 in conveying steel orother ferromagnetic sheets advantageously minimizes the likelihood thatthe sheets 12,14 will move out of alignment or slide off of theconveyors 20,22 as they are moved.

FIGS. 1 and 3 show the YAG laser as including a laser head (hereinaftergenerally referred to as the laser 40) which is movably mounted on agantry robot 44 provided on an overhead support 46. The support 46 issuspended at each of its ends by a pair of horizontal steel beams48a,48b in a position extending generally parallel to the axis A₁ andconveyors 20,22. The laser 40 is moved along the support 46 in thedirection of axis A₁ and above the seam line 16 by the robot 44. Therobot 44 is driven in movement through the engagement of a motor drivenpinion with a rack (not shown) extending horizontally along the support46. The rack extends a distance along the support 46 so as to permitmovement of the laser 40 along approximately 3 meters of the support 46.In this manner, a laser beam 56 (FIG. 7) may be emitted along apredetermined longitudinally extending pathline which is aligned withthe seam line 16 to be formed.

FIGS. 4 to 7 show best the sheet clamping units 36,38 which are used tomaintain the sheets 12,14 in a fixed position within the apparatus 10during welding by the laser 40. The clamping units 36,38 areindependently operable with unit 36 activatable to first clamp arespective sheet 12, and unit 38 activatable to clamp sheet 14. Clampingunit 36 and clamping unit 38 have essentially the identical constructionwith like reference numbers used to identify like components. Forclarity and brevity, clamping unit 36 is described in detail herebelow.It is to be appreciated from FIGS. 4 to 7 that clamping unit 38 isprovided having a mirror structural arrangement to clamping unit 36 andoperates in a like manner.

Clamping unit 36 includes an elongated electrically operated magnet 54about 0.3 meters wide and 3 meters in length which extendslongitudinally parallel to and adjacent the axis A₁, and a number steelclamping shoes 55 provided above the magnet 54 at longitudinally spacedlocations. FIG. 4 shows best the electromagnet 54 as having a flathorizontal upper surface 58. The electromagnet 54 shown in FIGS. 1 to 7has a straight elongated structure and is formed by joining a number ofgenerally rectangular magnet modules 53 end to end in a straight line.If desired, the modular construction of the electromagnet 54advantageously also enables the electromagnet 54 to curve. In thismanner, the electromagnet 54 may also be positioned following curved orbent seam lines, by incorporating one or more curved modules 53 or bysimply rearranging the positioning of the modules 53. The modules 53each consist of a permanent magnet which contains an electric coil. Whenelectrically pulsed, the coils in the modules 53 produce variablemagnetic fields to provide the required downward magnetic forces alongthe length of the electromagnet 54. The use of modules 53 in theconstruction of the magnet 54 advantageously enables the modification ofthe clamping units 36,38 to achieve optimal clamping of different shapedblanks.

The upper surface 58 of the electromagnet 54 acts both as a support uponwhich an associated one of the sheets to be joined rests, and as ahold-down mechanism for retaining the clamping shoes 55 in asheet-clamping position. The electromagnet 54 extends along a horizontalsupport beam 57. The beam 57 in turn is mounted at each of its ends totwo vertical beams 64a,64b (see FIG. 3) which are spaced apart asufficient distance to enable the sheets 12,14 and finished workpiece 18to move substantially unhindered therebetween. Although not essential, aservo-motor drive unit 66 is preferably also provided to verticallyraise or lower the beam 57 and electromagnet 54 of clamping unit 38 onthe beams 64a,64b, relative to the electromagnet 54 of clamping unit 36.It is to be appreciated, that the servo-motor drive unit 66advantageously permits vertical adjustment in the relative positioningof the sheets 12,14, as for example, to permit sheets of differentthicknesses to be welded together.

In the embodiment shown in FIG. 3, nine identical steel shoes 55 areprovided in each clamping unit 36,38. The shoes 55 are each formed fromcast iron and extend laterally across almost the entire width of theelectromagnet 54, and longitudinally a distance of between about 0.2 to0.4 meters. Each of the shoes 55 includes a generally flat horizontalbottom surface 68 which, when the clamping units 36,38 are activated,moves against the upper surface of a sheet therein to sandwich itbetween the bottom of the shoe 55 and the electromagnet 54. A rearwardedge portion 70 of the shoe 55, spaced furthest away from the axis A₁and seam line 16 slopes upwardly from the bottom surface 68. As will bedescribed later, the upward slope of the rearward edge portion 70 actsto assist in flattening and directing the sheets 12,14 between therespective electromagnets 54 and shoes 55 as they are slid into positionmoving the edge portions 12',14' towards alignment with the seam line16.

The shoes 55 of each clamping unit 36,38 are mounted on associatedvertically movable pneumatic piston cylinders 72 positioned on sides ofthe overhead support 46. By activating the piston cylinders 72, theshoes 55 are vertically moved between a lowered position, where thebottom surface 68 of the shoe 55 engages the upper surface of a sheetthereunder, and a raised position spaced a distance above theelectromagnet 54, which is sufficient to enable movement of a sheetbetween the electromagnet 54 and shoes 55.

It is preferred that when the electromagnet 54 is not activated, theshoes 55 are automatically returned to the raised position, as forexample, by means of the servo-control of the cylinders 72, permittingsubstantially unhindered sliding movement of the sheets 12,14 betweenassociated shoes 55 and electromagnet 54; and whereby the activation ofthe electromagnets 54 deactivates the cylinders 72, resulting in thelowering of the shoes 55 to return to the lowered sheet engagingposition.

The shoes 55 are each provided with a generally horizontal flat uppershoulder 74 extending approximately one-half of its width. The uppershoulder 74 advantageously permits placement of one or more rectangulariron bars 76 or other ferromagnetic weights (seen best in FIG. 8) to beplaced on the shoe 55. The addition of iron bars 76 may advantageouslybe used to decrease magnetic reluctance with respect to theelectromagnet 54, where thinner sheets 12,14 are to be welded.

FIGS. 4 to 7 show best an elongated 0.25 inch wide laser dump 59 alignedbeneath the axis A₁ and recessed beneath the upper surfaces 58 of themagnets 54. The laser dump 59 is preferably copper-lined to absorbexcess YAG laser energy, and is liquid cooled by pumping cooling wateralong a longitudinally extending coolant channel 61 which extendsbeneath the bottom of the dump 59.

The locating pins 30 of the sheet positioning assembly are provided inalignment at longitudinally spaced locations under the support 46. Thepins 30 are located in the apparatus 10 with one circumferential edgeportion of each pin 30 tangentially aligned with the laser dump 59 andaxis A₁. In this manner, when the edge portion 12' of the steel sheet 12is moved against the pins 30, the edge portion 12' aligns with each ofthe laser dump 59, the pathline of an emitted laser beam 56 and thedesired seam line 16 of the workpiece 18. As seen best in FIGS. 4 to 7,each locating pin 30 is vertically movable within a correspondingpneumatically operated cylinder 60 which is secured to a vertical sideof the electromagnet 54 of clamping unit 36 by bolts 62. By theselective activation or deactivation of the associated cylinders 60, thepins 30 are moved between a raised position, where the pins 30 engageand contact the proximal edge portion 12' of sheet 12, and a loweredposition moved vertically a distance below the lower surface of thesheets 12,14.

The positioning rods 32,34 are provided in sets of ten rods on each sideof the axis A₁. Each set of rods 32,34 is used to move a respective oneof the sheets 12,14 to a position so that the proximal edge portions12',14' are aligned with the predetermined seam line 16. As will bedescribed later, the positioning rods 32,34 are activatable to engagethe non-proximate edge portions of the respective sheets 12,14 and slidethe sheets 12,14 to a position with their proximal edge portions 12',14'in abutting engagement under the path line of the laser beam 56 forwelding.

The overall operation of the laser 40, gantry robot 44 and clampingunits 36,38 may be performed manually, but more preferably amicroprocessor control 78 (see FIG. 2) is provided. The microprocessorcontrol 78 may be equipped with software whereby the laser 40, gantryrobot 44, and clamping unit 36,38 operations may be either pre-set ortaught for repeated workpiece 18 production.

The use of the apparatus 10 is best described with reference to FIGS. 2and 4 to 7. Initially, the servo-motor drive unit 66 is activated toraise or lower the electromagnet 54 of clamping unit 38 relative to theelectromagnet 54 of clamping unit 36. The clamping unit 38 is moved sothe upper surfaces 58 of the electromagnets 54 of both units 36,38 areat the desired relative vertical position, having regard to thethicknesses of the steel sheets 12,14 to be joined.

The two sheets 12,14 which are to be welded together along theirrespective proximal edges 12',14' are then placed on a respectiveconveyor 20,22 with the edge portions 12',14 each oriented towards theaxis A₁. The conveyors 20,22 are then operated either simultaneously orseparately, to move the sheets 12,14 in the direction of arrow 42 intothe apparatus 10.

The pneumatic cylinders 60 are next activated to raise the locating pins30 to a height so as to engage the proximal edge portion 12' of thesheet 12 as it rests on the surface of the electromagnet 54 of clampingunit 36.

The set of locating rods 34 are then individually extended, whereby therods 34 engage the non-proximal edge portions of the sheet 12. Theengagement of the rods 34 with the non-proximal edge portions of thesheet 12 slides the sheet 12 within the apparatus 10. The sheet 12 ismoved so that the lower surface of part of the sheet 12 adjacent theedge portion 12' rests upon and is supported by the surface 58 of theclamping unit 36 electromagnet, and the proximal edge portion 12' ismoved in the direction of arrow 80 (seen in FIG. 4) against the pins 30into alignment with the axis A₁ and seam line 16. As the sheet 12 movestowards the pins 30, the sheet 12 is moved between the upper surface 58of electromagnet 54 and shoes 55 of the clamping unit 36 and intoengagement with each of the locating pins 30. The engagement of the edgeportion 12' of sheet 12 with the pins 30 thereby aligns the edge portion12' precisely under the path line of the laser beam 56 and seam line 16in the optimum position for welding. In addition, the movement of thesheet 12 between the shoes 55 and electromagnet 54 of clamping unit 36acts to flatten the sheet 12, removing any warp or bends from the sheet12.

With the proximal edge portion 12' engaging each of the locating pins30, the electromagnet 54 of the clamping unit 36 is activated and theassociated shoes 55 are lowered onto the upper surface of the sheet 12.The lowering of the shoes 55 sandwiches the sheet 12 in place betweenthe shoes 55 and electromagnet 54. When activated, the increasedmagnetic attraction between the electromagnet 54 and its associatedshoes 54 pulls the shoes 55 downwardly onto the sheet 12 with asufficient force selected to preventing movement of the edge portion 12'out of alignment with the predetermined seam line 16. It is to befurther appreciated that where sheets of ferromagnetic materials areclamped in place, the magnetic attraction of the sheet 12 directly tothe supporting electromagnet 54 further assists in holding the sheet 12in place.

With the sheet 12 clamped in a welding position with the edge portion12' aligned with the seam line 16, the cylinders 60 are deactivated toretract the locating pins 30 to the lowered position shown in FIG. 6,wherein the pins 30 are moved beneath both the lower surfaces of thesheets 12,14, and the upper surface 58 of the electromagnet 54 of theclamping unit 38.

The locating rods 36 are then activated to engage the non-proximal edgesof sheet 14, and move the sheet 14 towards the axis A₁ in the directionof arrow 81 (shown in FIG. 6) so that the proximal edge portion 14' ofthe sheet blank 14 is moved into abutting engagement with the proximaledge portion 12' of sheet 12. The movement of the sheet 14 by thelocating rods 36, similarly slides the lower surface of part of thesheet 14 which is adjacent the edge portion 14' onto the electromagnet54 of clamping unit 38, so that it is supported thereby. Once the edgeportions 12',14' are positioned in the proper abutting engagement, theelectromagnet 54 of the clamping unit 38 is activated, and theassociated shoes 55 are lowered onto the upper surface of the sheet 14.As with clamping unit 36, the magnetic attraction of the shoes 55towards the electromagnet 54, pulls the shoes downwardly with sufficientforce as to substantially prevent further movement of the sheet 14.

With each of the sheets 12,14 clamped in abutting engagement by therespective clamping units 36,38, the computer control 78 automaticallyactivates the laser 40 and gantry robot 44, to move the head of thelaser 40 along the support 46 by the motor driven pinion engaging therack (not shown). The movement of the laser 40 thereto moves an emittedlaser beam 56 along the path line welding the proximal edge portions12',14' together along the predetermined seam line 16. By the length oftravel of the laser 40 along the support 46, weld seams may be formedwhich are 3 or more meters in length.

The use of the YAG laser 40 has several advantages over conventional CO₂lasers. In particular, the YAG laser 40 is more suited to weldingaluminum and aluminum alloys. The YAG laser additionally permits fasterweld times of 3 meters per minute and enables simplified fibre opticbeam delivery to a comparatively small laser head. In addition, toachieve effective butt welding with conventional CO₂ lasers, it isnecessary that the edge portions 12',14' of the blanks 12,14 have ahighly pre-finished, minor smooth surface. This has heretofore proven tobe a significant drawback in the large scale implementation of CO₂lasers in continuous butt-welding processes. In contrast, the YAG laser40 effectively welds sheets 12,14 having normal sheared edges, withoutrequiring specialized edge treatment of the sheets 12,14.

It is to be appreciated that as the sheets 12,14 are clamped in place bythe attraction of respective shoes 55 and electromagnets 54, there is noneed to provide the apparatus 10 with a cumbersome and expensive clampsupporting frame to absorb opposing clamping forces associated withconventional welding apparatus. Further, the permanent magnets used inthe modules 53 are selected so that when the electromagnet 54 is notactivated, the sheets 12,14 and workpiece 18 may be moved into and outof the apparatus 10.

Following welding, the electromagnets 54 of both clamping units 36,38are deactivated and the associated shoes 55 are raised by theirrespective piston cylinders 72 to unclamp the finished workpiece 18. Theconveyors 20,22 are then activated to simultaneously move the finishedworkpiece 18 out of the apparatus 10 in the direction of arrow 42 (shownin FIG. 2) while simultaneously moving two new sheets to be joinedtherein.

The foregoing construction advantageously permits sheets made ofnon-ferromagnetic materials, such as tin or aluminum, to be securelyclamped in place during welding, by their being sandwiched between themagnetically attracted associated shoes and electromagnets.

While the YAG laser 40 is disclosed as being moved along the support 46by the engagement of a rack by a motor driven pinion, other drivemechanisms to be used with the gantry robot 44 are also possible andwill become apparent. Depending on the workpiece 18 to be formed, thelaser 40 may be operated to emit a continuous laser beam 56 whichperforms continuous welding along the whole of the seam line 16, oralternately, the laser 40 may be pulsed to form welds at longitudinallyspaced locations. Similarly, while the laser 40 disclosed with referenceto FIGS. 1 to 7 is preprogrammed to travel along a predetermined seamline, the laser could also include a seam line sensor whichautomatically senses the spacing between two sheet blanks and guides thelaser 40 in movement along the sensed seam line.

An alternate embodiment of the present invention is illustrated in FIGS.9 and 10 wherein like reference numerals are used to identify likecomponents. The apparatus 10 shown in FIGS. 9 and 10 has all of thecomponents as that shown in FIG. 1, however, a third magnetic steppingconveyor 85, two additional clamping units 86,88 and a second YAG laser90 are additionally provided. The clamping units 86,88, and laser 90 areused to weld not only sheets 12 and 14 together in the manner describedwith reference to FIGS. 4 to 7, but also weld an edge portion 92' ofthird steel sheet 92 to an edge portion 14" of steel sheet 14 which isremote from edge portion 14'.

The apparatus 10 welds the sheet 92 to sheet 14 along a second seam line94, aligned with an axis A₂ which is parallel to axis A₁ and seam line16. As in the embodiment shown in FIGS. 1 to 7, servo-motor drive units66 are provided to permit not only vertical adjustment of the height ofthe electromagnet 54 of unit 38 relative to that of unit 36, but alsovertical adjustment of the electromagnet 54 of units 86 and 88. Thelaser 90 is movably provided on a second overhead support 91 whichextends parallel to support 46 in essentially the same manner as laser40.

In use of the apparatus 10 of FIG. 9 and 10, the sheet blanks 12 and 14are joined together in the identical manner as that described withreference to FIGS. 4 to 7. Following the welding of sheets 12,14, thethird conveyor 85 is operated to move the sheet 92 in the direction ofarrow 42 into the apparatus 10, so that the edge portion 92' of thesheet 92 which is proximate to edge portion 14" is positioned facingtheretowards. The clamping unit 86 is activated to clamp the sheet 14 inthe same manner as clamping units 36,38. In the same manner as clampingunit 38, clamping unit 86 is activated to clamp the sheet 14, with sheet12 having been welded thereto, in position with the edge portion 14"aligned with the predetermined seam line 94. The third sheet 92 is thenmoved so that its proximal edge portion 92' which is proximate edgeportion 14" is positioned in abutting relationship therewith. Once thesheets 14,92 are so positioned, the fourth clamping unit 88 is activatedto clamp the sheet 92 in position with the edge portions 14',92'abutting and aligned with seam line 94. The second laser 90 is thenactivated and moved by a gantry robot along the support 91 to weldsheets 14 and 92 together along the second seam line 94.

It is contemplated that the sheet positioning assembly shown in FIGS. 9and 10 could also include additional locating pins and/or positioningrods (not shown) to assist in orienting the proximate edge portions 14"and 92' in the correct alignment with the seam line 94. Where a secondset of locating pins is used, it is to be appreciated that following thewelding of the sheet 12 to the sheet 14, the clamping units 36,38 aredeactivated and the locating rods 34,36 further adjusted to move thewelded sheets 12,14 to a corrected position prior to welding sheet 92.

While FIGS. 9 and 10 show the apparatus 10 as including two lasers 40,90which each travel along parallel path lines, the invention is not solimited. Depending on the workpiece which is to be produced, additionallasers and clamping units could also be provided. The lasers 40,90 mayalso be movable along perpendicular horizontal and/or verticaldirections, or alternately along curved or irregular non-linear pathlines, as for example, to produce workpieces having curved or irregularseam lines or the like.

Another embodiment of the present invention is illustrated in FIGS. 11and 12, wherein like reference numerals are used to identify likecomponents. FIGS. 11 and 12 show the apparatus 10 used in a productionline 100 for the continuous manufacture of composite workpieces 18.

The production line 100 is configured for the concurrent manufacture oftwo completed workpieces 18a,18b, and includes a single laser 40, whichis configured for 3 axis movement. As seen best in FIG. 12, the entirelaser 40 and support 124 is movable in a first horizontal direction viaa gantry robot 44 along an overhead support 46 and a slave support 146.As in the embodiments shown in FIGS. 1 to 7, the laser 40 is movable viaa gantry robot 44 along a rack provided on overhead support 46. Thesupports 46,146 are further slidable in a second horizontal directionperpendicular to the first on parallel spaced end supports 114a,114bwhich support each end of the support 46, and slave support 146. Aservo-drive motor 120 is provided at the end of support 46 engaging arack 122 extending along one support 114 thereby permitting the laserbeam 56 movement in any horizontal direction. In this manner, by movingthe laser 40 along the support 46, and moving the support 46 alongsupports 114a,114b, the laser beam 56 may be moved along substantiallyany path. In addition, it is preferable that the laser 40 be verticallymovable, thereby permitting its movement along all three axes andprovide increased adaptability to the apparatus 10.

In use, pairs of component sheets 12a,14a and 12b,14b are movedsequentially via robotic vacuum lifts 106a,106b from respective supplystacks 108a,108b, and are positioned on parallel magnetic feed conveyors110. The robotic vacuum lifts 10a,106b are used to move each componentsheet 12a,14a and 12b,14b, respectively through an initial qualifyingprocedure, to ensure correct initial positioning of the sheets on thefeed conveyors 110. In this regard, sets of locating pins 130, which aresubstantially identical to pins 30, are provided at spaced locationsbetween the conveyors 110 to position the sheets in the desired initialposition.

Each robotic vacuum lift 106a,106b operates with suction cups used toinitially pick up the sheets 12 and 14 under a high vacuum pressure, soas to fixedly retain each sheet as it is raised from the supply stack108. As the sheet is moved to a position immediately above the conveyors110, the suction pressure used to hold the sheet is reduced. Thereduction suction pressure is chosen so that the retained sheetcontinues to be suspended by the vacuum force of the lift 106, whilepermitting the sheet to be slid laterally relative to the vacuum lift106. The lift 106 is then moved to bring the retained sheet against oneset of pins 130 which have been extended above the top surface of theconveyors 110. By moving the edges of the sheets against the pins 130,the sheets may be slid on the lift 106 into the desired initial positionon the conveyors 110. Following the positioning of the sheets, thevacuum lift 106 is deactivated to release the sheet, and the pins 130are lowered to permit the sheets to be conveyed for welding.

Once the respective pairs of sheets 12a,14a and 12b,14b are in thedesired initial position, the feed conveyors 110 move the pairs ofsheets 12a,14a and 12b,14b in the direction of arrow 42, into a laseroperations room 112 in which the apparatus 10 is housed. The laseroperations room 112 provides an added safety feature whereby plantworkers are shielded by the room 112 from YAG laser energy emitted bythe laser 40. In this regard, the room may be provided with mail box orsliding doors (not shown) which close to optically isolate the apparatus10 during welding operations.

The conveyors 120 in turn convey the respective pairs of componentsheets 12a,14a and 12b,14b into respective clamping units 36a,38a and36b,38b which straddle one of the parallel paths along which the lasermoves.

The blanks 12a,14a and 12b,14b are positioned in the clamping units36a,38a and 36b,38b in the same manner as described in FIGS. 1 to 7aligned on a respective pair of electromagnets 54. In the mannerdescribed, once positioned, the sheets 12a,14a and 12b,14b are securedin place by clamping shoes 55 (FIG. 12). The laser 40 is then activatedto first weld blank 12a to blank 14a, and then by moving the laser 40and supports 46,146 relative to supports 114, to weld blank 12b to blank14b.

The feed conveyors 110 may be used to simultaneously feed the pairs ofsheets 12a,14a and 12b,14b into the laser operations room 112 and ontoaligned magnetic conveyors 120. Preferably, however, the production line100 is computer controlled so that the right half of the line 100, whichis shown as the top half of FIG. 11, operates independently of the lefthalf. In this manner, the conveyors 100 may be used to independentlyconvey the pairs of sheets 12a,14a and 12b,14b into laser operationsroom 112. The applicant has appreciated that independent left and rightoperations advantageously enable maximum production efficiency. Inparticular, while clamping units 36,38 are used to position one pair ofsheets 12a,14a, the laser 40 can be used to weld the other pair 12b,14balong the seam line.

Once the completed workpieces 18a,18b have been formed, the conveyors120 move the workpieces 18a,18b out of the laser operations room 112onto aligned exit conveyors 118 which convey the composite workpieces18a,18b to robotic lifts 125a,125b for placement in finished compositestacks 127a,127b.

FIG. 12 shows the shoes 55 as being mounted on a second steel beam 119,which is spaced above a corresponding electromagnet 54, and below thesupport 46. The shoes 55 are movably coupled to the beam 119 by pistons(not shown) in essentially the same manner as the coupling of the shoes55 to the support 46 shown in FIGS. 4 to 7. The electromagnets 54 andbeams 119 are movable along respective slides 121,123. The slides 121,123 extend perpendicularly to each respective magnet 54 and beam 119 andallow adjustment of the lateral positioning of the clamping units 36,38,for welding different workpieces 18.

A further embodiment of the present invention is illustrated in FIGS. 13and 14 wherein like reference numerals are used to identify likecomponents. FIG. 13 shows a component production line 100 adapted forthe simultaneous manufacture of completed composite workpieces 18a-fusing a laser 40 which is rotatably mounted within an overhead support124 shown best in FIG. 14. As in the embodiment shown in FIG. 11, thelaser 40 is adapted for 3 axis movement on supports 46,146 and114a,114b. The laser 40 is, however, additionally provided on an armassembly 126 which is journalled in rotational movement along an inneredge portion of a hollow supporting cylinder 130.

The production line of FIG. 13 operates in a similar manner to thatshown in FIG. 11, with the exception that six pairs of component sheets12a,14a, 12b,14b, 12c,14c, 12d,14d, 12e,14e, 12f,14f are welded withinthe laser operations room 112 at any one time. In addition, with theapparatus 10 shown in FIG. 13 welding of the sheets 12,14 occurs along acurved line. In this regard, the configuration of the magnet modules 53and shoes 55 are modified to reflect the curved path of the laser beam56.

In the production line FIG. 13 and 14, a single laser 40 may be used tosequentially weld each pair of sheet components 12,14 or, additionallasers (shown in phantom) may further be provided. The path of laserbeam 56 movement may be preprogrammed to follow a precise repetitivepath which is preset having regard to the blank 12,14 configuration andthe weld seam 16. More preferably, however, the laser 40 is providedwith a seam tracking optic sensor which automatically follows anyspacing between the blanks 12,14 and centres the laser beam 56 thereonto perform welding operation.

While FIG. 3 illustrates the clamping unit 36 as including a number ofindividual shoes 55 spaced longitudinally over a correspondingelectromagnet 54, fewer larger, or even a single elongated shoe couldalternately be provided without departing from the scope of the presentinvention.

Although the preferred embodiment of the invention discloses the use ofa YAG laser to weld the sheet blanks together, depending on the sheetmaterial to be joined, other welding apparatus, including plasma arc orelectron beam welding apparatus, arc welding apparatus or the like couldalso be used and will now become apparent.

The use of cast iron shoes 55 advantageously provide an economical andeasily manufactured construction which has a high degree of magneticsusceptibility, so as to maximize the magnetic attraction between theshoe 55 and electromagnet 54. If desired, however, the shoes may also beformed from steel or other ferromagnetic materials. Alternately, inanother possible construction, the shoe may itself be provided with anelectromagnetic insert or coil for inducing a magnetic polarity thereinwhich is opposite to that of the underlining portion of the supportingelectromagnet.

While the preferred embodiment describes the use of the presentinvention in butt welding two or more sheets 12,14 other applicationsare also possible. For example, by inclining the laser beam relative tothe vertical axis, the present invention may also be used to mash weldtwo or more sheet blanks together.

Although FIGS. 4 to 7 show the locating pins 30 as being movable in acylinder 60 secured to electromagnet 54, other constructions are alsopossible. The pins 30 could, for example, be mounted to a supportsuspended above one or both magnets 54, and move downwardly into boresformed in the upper magnet surface 58. In such a configuration themagnets 54 would be separated only by the laser dump 59 andadvantageously support the portions of the blanks 12,14 closest to theedge portions 12',14' to be welded.

While the detailed description of the invention describes andillustrates preferred embodiments of the invention, the invention is notso limited. Many modifications and variations will now appear topersons. skilled in this art. For a definition of the invention,reference may be had to the attached claims.

We claim:
 1. An apparatus for forming a workpiece by aligning and buttwelding together in an abutting configuration proximal edge portions offirst and second component sheets, said apparatus including,weldingmeans for welding the proximal edge portion of the first sheet to theabutting proximal edge portion of the second sheet along a seam line,positioning means for positioning the proximal edge portion of saidfirst sheet in a welding position, first sheet supporting means forsupporting said first sheet with said edge portion of said first sheetin said welding position, the first sheet supporting means including,first clamping means for releasably retaining said first sheet in saidapparatus during welding, the first clamping means comprising, firstmagnetic hold-down means for engagingly supporting a first side of saidfirst sheet, said first magnetic hold-down means comprising a series ofelectronically switchable permanent magnets arranged in an elongatedarray, and first shoe means for engaging the second other side of saidfirst sheet, the first shoe means characterized by magneticsusceptibility and being movable relative to said first hold-down meansbetween a forward position, wherein said first shoe means is movedtowards said first hold-down means to substantially prevent movement ofsaid first sheet therebetween, and a rearward position, wherein saidfirst shoe means is moved away from said first hold-down means to permitsubstantially unhindered movement of said first sheet therebetween, thefirst clamping means being activatable to produce a magnetic field inthe first hold-down means sufficient to magnetically attract the firstshoe means to the forward position and clamp the first sheettherebetween, and second sheet supporting means for supporting saidsecond sheet with said proximal edge portion of said second sheet in agenerally abutting relationship with said proximal edge portion of saidfirst sheet.
 2. An apparatus as claimed in claim 1 wherein said secondsheet supporting means includes,second clamping means for releasablyretaining said second sheet in said apparatus during welding, the secondclamping means comprising, second electromagnetic hold-down means forengagingly supporting a first side of said second sheet, and second shoemeans for engaging the second other side of said second sheet, thesecond shoe means characterized by magnetic susceptibility and beingmovable relative to said second hold-down means between a forwardposition, wherein said second shoe means is moved towards said secondhold-down means to substantially prevent movement of said second sheettherebetween, and a rearward position, wherein said second shoe means ismoved away from said second hold-down means to permit substantiallyunhindered movement of said second sheet therebetween, wherein thesecond clamping means is activatable to produce a magnetic field in thesecond hold-down means sufficient to magnetically attract the secondshoe means to the forward position to clamp the second sheettherebetween.
 3. An apparatus as claimed in claim 2 wherein said firstand second supporting means support said respective first and secondsheets thereon in a substantially horizontal orientation,saidpositioning means including at least one locating pin vertically movablebetween a first sheet engaging position, wherein said pin is positionedrelative said first supporting means to engage the proximal edge portionof said first sheet thereon, and a second position wherein said pin ismoved relative to said second supporting means whereby the edge portionof said second sheet may be moved into direct engagement with the edgeportion of said first sheet.
 4. An apparatus as claimed in claim 3wherein said first and second support means are adjustable verticallyrelative to each other.
 5. An apparatus as claimed in claim 1 whereinsaid first support means includes magnetic indexing conveyor means formovably supporting said first sheet thereon.
 6. An apparatus as claimedin claim 3 wherein said second shoe means comprises an elongated steelshoe and said second hold-down means comprises an elongatedelectromagnet, each of said shoe and said electromagnet extendinglongitudinally generally parallel to and adjacent said seam line.
 7. Anapparatus as claimed in claim 3 wherein said first shoe means comprisesa plurality of shoe members spaced in a longitudinal direction generallyparallel to and adjacent said seam line.
 8. An apparatus as claimed inclaim 3 wherein said welding means comprises a yttrium aluminum garnetlaser.
 9. An apparatus as claimed in claim 7 wherein each of said shoemembers comprise generally planar upper and lower surface portions,andsaid apparatus further includes a plurality of steel plates removablycoupled to the upper surface of an associated one of said shoe members.10. An apparatus for forming a workpiece by aligning and butt weldingtogether in an abutting configuration proximal edge portions of firstand second sheet blanks, said apparatus including,welding means forwelding the proximal edge portion of the first sheet blank to theproximal edge portion of the second sheet blank along a seam line,positioning means for positioning the proximal edge portion of saidfirst sheet blank in a welding position, first sheet supporting meansfor supporting said first sheet blank with said edge portion of saidfirst sheet blank in said welding position, the first sheet blanksupporting means including, magnetic indexing conveyor means for movablysupporting said first sheet blank thereon, first clamping means forreleasably retaining said first sheet blank in said apparatus duringwelding, the first clamping means comprising, first magnetic hold-downmeans for engagingly supporting a first side of said first sheet blank,and first shoe means for engaging the second other side of said firstsheet blank, the first shoe means characterized by magneticsusceptibility and being movable relative to said first hold-down meansbetween a forward position, wherein said first shoe means is movedtowards said first hold-down means to substantially prevent movement ofsaid first sheet blank therebetween, and a rearward position, whereinsaid first shoe means is moved away from said first hold-down means topermit substantially unhindered movement of said first sheet blanktherebetween, the first clamping means being activatable to produce amagnetic field in the first hold-down means sufficient to magneticallyattract the first shoe means to the forward position and clamp the firstsheet blank therebetween, and second sheet blank supporting means forsupporting said second sheet blank with said proximal edge portion ofsaid second sheet blank in a generally abutting relationship with saidproximal edge portion of said first sheet blank.
 11. An apparatus asclaimed in claim 10 wherein said second sheet supporting means includessecond clamping means for releasably retaining said second sheet blankin said apparatus during welding, the second clamping meanscomprising,second electromagnetic hold-down means for engaginglysupporting a first lower side of said second sheet blank, and secondshoe means for engaging the other upper side of said second sheet blank,the second shoe means characterized by magnetic susceptibility, andbeing vertically movable relative to the second hold-down means betweena sheet engaging position, wherein the second shoe means is loweredtowards the second hold-down means to engage and clamp the second sheetblank between the second shoe means and second hold-down means, and arelease position wherein the second shoe means is raised above thesecond hold-down means a distance selected to permit movement of thesecond sheet blank therebetween, wherein the second clamping means isactivatable to produce a magnetic field in the second hold-down means tomagnetically attract the second shoe means and assist in moving thesecond shoe means to the sheet engaging position to clamp the secondsheet blank therebetween.
 12. An apparatus as claimed in claim 11whereinsaid positioning means including a locating pin having a sheetengaging circumferential portion aligned with said seam line, said pinbeing vertically movable between a first sheet engaging position,wherein said pin is moved vertically relative to said first supportingmeans so that the circumferential portion engages the proximal edgeportion of said first sheet, and a second lower position wherein saidpin is lowered relative to said first sheet to a position wherein theedge portion of the second sheet may be moved into direct engagementwith the edge portion of the first sheet.
 13. An apparatus as claimed inclaim 12 wherein said first and second support means are adjustablevertically relative to each other.
 14. An apparatus as claimed in claim12 wherein said welding means comprises a yttrium aluminum garnet laserhaving a laser head which is movable along a path substantially alignedwith said seam line.
 15. Use of an apparatus to form a workpiece bywelding together proximal edge portions of first and second sheetblanks, said apparatus including,welding means for welding the proximaledge portion of the first sheet to the proximal edge portion of thesecond sheet along a seam line, positioning means for positioning theedge portion of said first sheet in a position substantially alignedwith said seam line, first sheet supporting means for supporting saidfirst sheet with said edge portion of said first sheet in said positionaligned with said seam line, the first sheet supporting means including,first clamping means activatably to releasably retain said first sheetin said apparatus during welding, the first clamping comprising, firstelectromagnetic hold-down means for engagingly supporting a first sideof said first sheet, and first shoe means for engaging the second otherside of said first sheet, the first shoe means having magneticsusceptibility and being movable relative to said first hold-down meansbetween a forward clamping position wherein said first shoe means ismoved towards said first hold-down means to engage and clamp a portionof said first sheet therebetween, and a rearward position wherein saidfirst shoe means is moved away from said first hold-down means adistance sufficient to permit movement of said first sheet therebetween,and second sheet supporting means for supporting said second sheetthereon with said edge portion of said second sheet in an approximatelyabutting relationship with said edge portion of said first sheet, saidsecond sheet supporting means including, second clamping meansactivatable to releasably retain said second sheet in said apparatusduring welding, the second clamping means comprising, secondelectromagnetic hold-down means for engagingly supporting a first sideof said second sheet, and second shoe means for engaging the secondother side of said second sheet, the second shoe means having magneticsusceptibility and being movable relative to said second hold-down meansbetween a forward clamping position, wherein said second shoe means ismoved towards said second hold-down means to engage and clamp a portionof said second sheet therebetween, and a rearward position wherein saidsecond shoe means is moved away from said second hold-down means adistance sufficient to permit movement of said second sheettherebetween, wherein with each of said first and second meanspositioned in said respective rearward positions, said workpiece isformed by the steps of, moving said first sheet between said first shoemeans and said first hold-down means to engage said positioning meansand to position said edge portion of said first sheet substantially inalignment with said seam line, activating said first clamping means toproduct a magnetic field in said first hold-down means and move thefirst show means to the forward position to retain the first sheettherebetween, moving said second sheet between said first shoe means andsaid second hold-down means to position the edge portion of the secondsheet in a position abutting the edge portion of the first sheet,activating the second clamping means to produce a magnetic field in saidsecond hold-down means and move the second shoe means to the forwardposition to retain the second sheet therebetween, and activating saidwelding means to perform welding along the seam line to weld togetherthe edge portion of the first sheet and the edge portion of the secondsheet.
 16. Use of the apparatus as claimed in claim 15 wherein saidpositioning means includes a locating pin, the locating pin including afirst sheet engaging circumferential position aligned with said seamline,the pin being vertically movable between a first sheet engagingposition, wherein said pin is moved vertically relative to said firstsupporting means so that the circumferential portion engages theproximal edge portion of said first sheet, and a second lower positionwherein said pin is lowered relative to said first sheet to a positionwherein the edge portion of the second sheet may be moved into directengagement with the edge portion of the first sheet, and the step ofmoving said first sheet includes the step of moving the edge portion ofthe first sheet into abutting contact with the circumferential portionof the locating pin, and immediately preceding the step of moving thesecond sheet, lowering the locating pin to the second lower position.17. An apparatus as claimed in claim 5 further including,suction liftmeans for lifting said first sheet onto said conveyor means, saidsuction lift means being selectively operable in a high pressure vacuummode wherein said first sheet is fixedly retained by said lift means,and a reduced pressure vacuum mode wherein said first sheet is slidableretained thereby, and guide means for guiding said first sheet in adesired position on said conveyor means when said lift means is operatedin said reduced pressure vacuum mode.
 18. Use of the apparatus asclaimed in claim 15, wherein said seam line is a predetermined seamline.
 19. Use of the apparatus as claimed in claim 15, wherein saidwelding means comprises a yttrium aluminum garnet laser.
 20. Anapparatus as claimed in claim 10 further including,suction lift meansfor lifting said first sheet blank onto said indexing conveyor, saidsuction lift means being selectively operable in a high pressure vacuummode wherein said first sheet blank is fixedly retained by said liftmeans, and a reduced pressure vacuum mode wherein said first sheet isslidable retained thereby, and guide means for guiding said first sheetblank in a desired position on said conveyor when said lift means isoperated in said reduced pressure vacuum mode.